The present invention is directed to signal conversion apparatuses, and especially to signal conversion apparatuses for converting fully-differential class-AB input signals to rail-to-rail single-ended output signals.
It is desirable that operational amplifiers for low supply voltages be able to utilize the largest portion of the supply voltage range possible, especially in terms of output signal, in order to improve the signal-to-noise ratio. In many arrangements base-emitter or gate-source voltages prevent the output signal from reaching within about 0.7 volts of the positive or negative supply voltage (i.e., the top or bottom rail). This limitation in output signal swing becomes a problem when the supply voltage is low, as is the case for example in battery powered systems or other low power applications.
Two-stage gain amplifiers have been used to provide rail-to-rail operation of output devices. Such an arrangement offers good linearity for larger output swings, but requires Miller compensation in the form of a feedback capacitor between the output and input of the single-ended signal output device. Miller capacitance is usually on the order of several picofarads and effects pole splitting to contribute to stable operation of the circuit. The value of the Miller compensation capacitance may be reduced by including a resistor in series (or a MOSFET in triode operation) with the Miller capacitance. Such an additional resistor has the effect of moving the right semi-plane zero to the left side of the origin in a pole plot and speeds up the operation of the apparatus. However, the value of the resistor must often be the inverse of output transistor's transconductance and may therefore not be practical in some applications where the amplifier is driving heavy loads.
The single-ended signal output device may be speeded up by employing a faster input, such as by employing a Class AB input device. A Class AB amplifier, for example, maintains current flow at all times so that an output device used with the Class AB amplifier can begin operation nearly instantly without lags that are present when operating with other types of amplifiers. Prior art employment of a Class AB input stage to a single-ended signal output device has typically used only one of the outputs presented by the Class AB input stage. That is, while the Class AB amplifier, or input stage, is known to provide a faster input section for any output stage, its speed (or slew rate) improvement is still limited because the input has to charge or discharge the Miller capacitance seen by the high impedance input node to the output stage.
There is a need for an apparatus and method for converting a fully-differential input signal varying between a top rail and a bottom rail that can produce a rail-to-rail single-ended output signal representative of the differential input signal.